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Topic: Help with power supply (Read 569 times) previous topic - next topic

Wazzled

May 05, 2011, 02:53 am Last Edit: May 05, 2011, 03:17 am by Wazzled Reason: 1
Hi, I'm going to make an RGB LED table.  

I plan on using 2x tlc5940 and multiplex them.

My questions is as it will be an 8x8 grid of RGB LEDs giving 192 individual LEDs at 25ma each will I need a 4.8A+ powersupply or does multiplexing mean I only need a 600ma supply as only one row will be lit up at a time.

EDIT: Just noticed that power related stuff is in a different section.  Can a mod please move.

Thanks

Jonny

westfw


Wazzled

Thanks, should be easy enough to get a power supply for that then.  Was thinking I needed around a 5A and was finding it difficult to find a 5v one for a sensible price.

CrossRoads

Look at the 2nd item here
http://www.mpja.com/products.asp?dept=37&main=1
I use this in a lot of my projects.
25mA is really pushing it for most LEDs as well. Dropping to 20mA is more realistic.  With multiplexing teh average current drops a lot as you've noted.

Designing & building electrical circuits for over 25 years.  Screw Shield for Mega/Due/Uno,  Bobuino with ATMega1284P, & other '328P & '1284P creations & offerings at  my website.

BenF


My questions is as it will be an 8x8 grid of RGB LEDs giving 192 individual LEDs at 25ma each will I need a 4.8A+ powersupply or does multiplexing mean I only need a 600ma supply as only one row will be lit up at a time.

If you multiplex 192 LED's with two 16-channel drivers, this suggests a 1/6 duty cycle (192 / 32).  With a 600mA power supply, perceived light intensity will then equal the equivalent of 3.12mA per LED. To get the full potential from your LED's, you need to feed each LED 6 times the rated current or about 120mA each (this is also max drive current for the tlc5940). For this, you need a DC power adapter with a minimum output of 3.84A (120mA * 32).

Wazzled

#5
May 06, 2011, 01:41 am Last Edit: May 06, 2011, 02:01 am by Wazzled Reason: 1

If you multiplex 192 LED's with two 16-channel drivers, this suggests a 1/6 duty cycle (192 / 32).  With a 600mA power supply, perceived light intensity will then equal the equivalent of 3.12mA per LED. To get the full potential from your LED's, you need to feed each LED 6 times the rated current or about 120mA each (this is also max drive current for the tlc5940). For this, you need a DC power adapter with a minimum output of 3.84A (120mA * 32).


As an electronics noob that has went completely over my head.  Now I'm confused.

Thanks for the link crossroads but im in the UK.  I have a 5v 2A supply here that I was gonna use but now it seems I can't.


westfw

In theory, if you multiplex your LEDs, they are only "on" for 1/N of the time, and if you want them to use the same "average" power, you have to provide N times the current for the time that they are on.

Unfortunately, it's more complicated than that.  Most LEDs won't put out N times the optical brightness if they are "overdriven" with N times their nominal maximum rating.  In fact, that might not be good for the LEDs.  There may or may not be a separate spec in the datasheet for pulsed power, but it isn't safe to assume that an LED rated for 25mA continuous current can be pulsed at six times that current for 1/6th of the time.

Fortunately, there are also some complex psychological effects that end up meaning that "perceived brightness" for a pulsed LED may have more to do with peak brightness rather than average brightness, at least under some circumstances...

I would say to go ahead and build your circuit with the 2A power supply, and adjust the software or hardware configuration so that you are not exceeding that 2A current.  You'll probably be happy with the brightness.  If not, THEN you can go looking for a beefier power supply (Jameco has some 5V 5A supplies for about $20...)

BenF

#7
May 06, 2011, 11:46 am Last Edit: May 06, 2011, 11:49 am by BenF Reason: 1
For this project I think it is fair to say that a 600mA supply is likely to be underrated and so a second opinion on the first advice was appropriate.


Fortunately, there are also some complex psychological effects that end up meaning that "perceived brightness" for a pulsed LED may have more to do with peak brightness rather than average brightness, at least under some circumstances...

I will not try to pose as an expert on human vision or "psychological effects" by any standards, but I think there is less to the above than meets the eye, literally so to speak. It is generally quoted that apparent brightness of a pulsed LED is more determined by peak actual brightness rather than average actual brightness. This is true under some circumstances, but mainly due to physics rather than human vision or "psychological effects". Understanding why and what those circumstances are might help with LED based lighting designs.

A LED pulsed at 20mA with a 10% duty cycle will typically appear much brighter than the same LED with a steady DC of 2mA. The main reason for this however is physics, not psychology. A LED as a light emitter is simply less efficient at low currents and so a pulsed higher current will produce more brightness given the same average power.

There appears to be general consensus that human vision is non-linear and that a small increase in actual brightness will result in a relatively higher increase in perceived brightness. So far so good, but when it comes to time-integration, peak brightness plays a very small part whereas average brightness correlates well with perceived brightness. So for a multiplexed design, average power is the dominant perceived brightness factor as long as you keep the LED operating at or near peak efficiency.

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